There are many known types of catalytic reactors. For example, catalytic reactors are common in fuel processing systems or subsystems, such as those that produce hydrogen. For example, proton exchange membrane (PEM) fuel cell systems will commonly include a fuel processing subsystem that produces hydrogen.
More specifically, in many PEM fuel cell systems, a fuel such as methanol, methane, or a similar hydrocarbon fuel is converted into a hydrogen-rich stream for the anode side of the fuel cell. In many systems, humidified methanol or natural gas (methane) and air are chemically converted to a hydrogen-rich stream known as reformate by a fuel processing subsystem of the fuel cell system. This conversion takes place in a reformer where the hydrogen is catalytically released from the hydrocarbon fuel. A common type of reformer is an Auto-thermal Reactor (ATR), which uses air and steam as oxidizing reactants. As the hydrogen is liberated, a substantial amount of carbon monoxide (CO) is created which must be reduced to a low level (typically less than 10 ppm) to prevent poisoning of the PEM membrane.
To reduce the CO concentration to within acceptable levels, several catalytic reactions will generally be used in the fuel processing subsystem to remove CO in the reformate flow. Typical reactions for reduction of CO in the reformate flow include a water-gas shift, as well as a selective oxidation reaction over a precious metal catalyst (with a small amount of air added to the reformate stream to provide oxygen) in a device commonly referred to as a selective oxidizer. Generally, several stages of CO cleanup are required to obtain a reformate stream with an acceptable CO level. Each of the stages of CO cleanup requires the reformate temperature to be reduced to relatively precise temperature ranges so that the desired ca-catalytic reactions will occur and the loading amount of precious metal catalyst can be minimized.
For example, the desired reaction during a selective oxidation process is [2 CO+O2→2 CO2+283 KJ/mol]. However, there are other competing reactions that are detrimental to the removal of CO from the reformate stream. Specifically, the other competing reactions are a hydrogen oxidation [H2+½O2→H2O+242 KJ/mol] which converts desired hydrogen gas into water, a reverse water-gas shift [CO2+H2+41 KJ/mol→H2O+CO] which creates additional harmful CO as well as depleting the amount of hydrogen gas, and methanations [CO+3H2→CH4+H2O+206 KJ/mol] and [CO2+4 H2→CH4+2 H2O+165 KJ/mol] which also deplete the amount of hydrogen gas in the reformate stream. The catalyst and initial temperature are chosen to favor the CO oxidation over the reverse water-gas shift and methanation. However, temperature fluctuations can cause the competing reactions to hinder CO removal performance. Furthermore, the optimum temperature for selective oxidation varies depending upon the concentration of carbon monoxide in the reformate. More specifically, the optimum temperature for selective oxidation typically tends to decrease as the concentration of carbon monoxide in the reformate decreases. Additionally, the activity of the catalyst, or the rate at which the desired reaction occurs, is a function of the concentration of the reactants (CO and O2) and temperature.
The CO oxidation reaction, the H2 oxidation reaction, as well as the methanation reaction are all exothermic, releasing heat as each respective reaction progresses. Therefore, the temperature of the reformate fluid stream can increase as much as 100° C. as it passes through a selective oxidation reactor even if the desired selective oxidation reaction initially dominates. As the temperature increases, the reaction selectivity for CO oxidation decreases with respect to the competing reactions, there-by decreasing overall CO removal efficiency. Thus, it is desirable to remove heat from the reformate flow as it is reacted so as to not lose selectivity of the reaction. However; during low temperature start up conditions, cooling of the reformate fluid stream in the catalytic reaction region can be undesirable because it reduces the already low activity of the catalytic reaction. In fact, it can be advantageous not to cool the reformate during a low temperature start up, because this would allow the catalyst to come up to temperature more quickly.